Japan, 1699. On a moonlit night in Ezogashima, the northernmost island of Japan, a woman is running through the forest when an arrow zooms out of the darkness to strike her dead. Meanwhile, a world away in the city of Edo, the eight-year-old son of Sano Ichiro, the samurai detective who has risen to power and influence in the shogun's court, vanishes during a moon-watching party.
When Sano's political rival, Lord Matsudaira, hints that the boy may be in Ezogashima, Sano's wife, Reiko, insists on accompanying him on the desperate journey. After an eleven-day voyage through cold and treacherous waters, they arrive at Ezogashima, only to find that Lord Matsumae, distraught at the murder of his mistress, is holding the whole province hostage until someone confesses to the crime. No one is allowed in or out of Ezogashima, and although Matsumae tells Sano his son is there, he refuses to release him.
Sano strikes a deal: He will solve the murder of Matsumae's mistress if Lord Matsumae will free the hostages and return their son. Soon, however, he and Reiko find themselves caught up in a dangerous scheme that includes clan warfare, jealous husbands, and murderous betrayal.